Experimental

Melting points were determined using a Mel-Temp 3.0 capillary melting point apparatus and are uncorrected. IR was recorded on a Perkin Elmer Spectrum One spectrometer.

Analytical TLC was performed on Whatman flexible silica gel plates containing fluorescent indicator on aluminum backing to monitor reactions. TLC visualization was done with UV light. 1_{H and} 13_{C NMR spectra were recorded using a Bruker Avance 400 instrument and the}

δ are in ppm relative to TMS and coupling constants are in Hertz. Typical proton and carbon NMR spectra are found in the appendix. Elemental analysis was obtained from Atlantic Microlab, Norcross, GA. Mass spectrometry was obtained from the GSU Mass Spectrometry Facility, Atlanta, GA. Reagents were purchased and used without further purification from Sigma- Aldrich, Acros Organics, Lancaster, Alfa Aesar, Frontier Scientific, and Carbocore.

2,5-Bis(4-cyanophenyl)thiazolo[5,4-d]thiazole (36)

4-Cyanobenzaldehyde 34 (7.05 g, 54 mmol), dithiooxamide 35 (3.01 g, 25 mmol), and DMF (50 ml, anhydrous) were added together under a nitrogen atmosphere with stirring. The mixture was heated to reflux. After 24 h at reflux, the red-orange mixture was cooled to room

overnight to yield crude 36 (4.85 g) as an orange solid. The crude sample was recrystallized in DMF (~1L). After 3 days, the mixture was filtered, washed with DMF, and dried overnight under vacuum to yield 36 (5.11 g, 15 mmol, 59%) as a yellow solid: mp 360.0-361.4o_{C, lit. 354-} 55oC; 1H NMR (100 MHz, CDCl3) δ (ppm) 8.10 (4H, d, J = 8.4Hz), δ 7.76 (4H, d, J = 7.6 Hz); Anal. Calcd for C18H8N4S2: C 62.77, H 2.34, N 16.27; Found C 62.75, H 2.32, N 16.25.

2,5-Bis(4-amidinophenyl)thiazolo[5,4-d]thiazole HCl Salt (37, DB 1929)

2,5-Bis(4-cyanophenyl)thiazolo[5, 4-d]thiazole (36) (0.34 g, 0.99 mmol) was added to THF (10 ml, anhydrous) under a nitrogen atmosphere with stirring at room temperature. Lithium bis(trimethylsilyl)amide (8.00 ml of 1.0 M in THF) was then added drop wise. The mixture was allowed to stir until completely dissolved. After 3 days, the flask containing the transparent red- brown solution was cooled to 00C for 30 min. The solution was made slightly acidic by the drop wise addition of an saturated EtOH-HCl solution (~3 ml) and allowed to stir in the ice bath overnight. Following overnight stirring, the yellow mixture was concentrated, suspended in anhydrous ether, and allowed to stir at room temperature for 30 min. The mixture was then filtered and washed with ether. The yellow solid was then dried under vacuum at 500C for 15 min.

The crude salt was then converted into the free base by suspending it in H2O (~1 ml), placing it in an ice bath, and adding NaOH (10%) dropwise until the solution was slightly basic. The orange mixture was then filtered with cold H2O and stored overnight in a vacuum desiccator. The resulting orange solid was suspended in a 50:50 EtOH-MeOH mixture and allowed to stir for 10 min. For conversion into the HCl salt, saturated EtOH-HCl (~1 ml) was added and the

mixture was allowed to stir overnight at room temperature. The precipitate was filtered and washed with ether and allowed to dry overnight in a vacuum desiccator to yield 37 (DB 1929)

(0.355 g, 0.738 mmol, 75%) a yellow solid: mp 354.2-356.4o_C; 1_{H NMR (100 MHz, d}

6-DMSO) δ (ppm) 9.34 (6H, s, NH), δ 8.29 (4H, d, J = 8.4 Hz); δ 8.03 (4H, d, J = 8.4 Hz); 13C NMR (400 MHz, d6-DMSO) δ (ppm) 168.2, 165.2, 152.4, 137.7, 130.6, 129.9, 127.0; MS-ESI calcd m/z 378.48, found m/z 379.2 [M+H]; Anal. Calcd for C18H14N6S2*2 HCl*1 H2O*0.25 CH3CH2OH: C 46.20, H 4.09, N 17.47; Found C 46.25, H 3.99, N 17.10.

2,5-Bis(4-cyano-2-fluorophenyl)thiazolo[5,4-d]thiazole (39)

4-Cyano-2-fluorobenzaldehyde 38 (1.62 g, 11 mmol) and dithiooxamide 35 (0.51 g, 4 mmol) were suspended in o-dichlorobenzene (15 ml) under a nitrogen atmosphere and heated at reflux with stirring. After 24 h at reflux, the mixture was cooled, filtered, and washed with cold EtOH. The resulting solid was then dried under vacuum to yield crude 39 (0.84 g) as a brown solid. The crude solid was then suspended in DMF, boiled for about 6 h, and filtered hot. Once cooled, crystallization was induced with the use of an ice bath. Four days later, the resulting solid was filtered and washed with cold DCM followed by being dried under vacuum with heat (~100o_{C) to yield 39 (0.47 g, 1.24 mmol, 29%) as brown solid: mp 384.3-386.8}o_C; 1_{H NMR} (100 MHz, d6-DMSO) δ (ppm) 8.439 (1H, t), δ 8.145 (1H, d, J = 11.6 Hz); δ 7.907 (1H, d, J = 7.6 Hz); IR (cm-1) 2240, 1564, 1118, 898, 801; Anal. Calcd for C18H6N4S2F2: C 56.83, H 1.59, N 14.73; Found C 56.79, H 1.60, N 14.52.

2,5-Bis(4-amidinophenyl -2-fluoro)thiazolo[5,4-d]thiazole HCl Salt (40)

2,5-Bis-(4-cyano-2-fluorophenyl)thiazolo[5, 4-d]thiazole (39) (0.40 g, 1.05 mmol) was added to THF (10 ml, anhydrous) under a nitrogen atmosphere with stirring at room temperature. Lithium bis(trimethylsilyl)amide (9.00 ml of 1.0 M in THF) was then added drop wise. The mixture was allowed to stir until completely dissolved. After a week, the mixture had not dissolved so a second addition of THF (5 ml) and lithium bis(trimethylsilyl)amide (10.00 ml) were added with continued stirring at room temperature. About 4 days later, a third addition of lithium bis(trimethylsilyl)amide (10.00 ml) was added. A week later a forth addition of lithium bis(trimethylsilyl)amide (12.00 ml) was added and allowed to continue stirring at room

temperature. After another 3 weeks or 7 weeks total, the mixture had become a red-brown solution. The flask containing the red-brown solution was then cooled to 00C for 30 min. The solution was made slightly acidic by the drop wise addition of a saturated EtOH-HCl solution (~4 ml) and allowed to stir in the ice bath overnight. Following overnight stirring, the mixture was concentrated, suspended in anhydrous ether, and allowed to stir at room temperature for 30 min. The mixture was then filtered and washed with ether. The light brown solid was then dried under vacuum at 500C for 15 min.

The crude salt was then converted into the free base by suspending it in H2O (~15 ml), placing it in an ice bath, and adding NaOH (10%) dropwise until the solution was slightly basic. The orange mixture was then filtered with cold H2O and stored overnight in a vacuum desiccator. The resulting brown solid was suspended in an 80:20 mixture of EtOH-MeOH (~18ml) and allowed to stir for 10 min. For conversion into the HCl salt, EtOH-HCl (~3 ml) was added until the suspension was slightly acidic and it was allowed to stir overnight at room temperature. The

precipitate was filtered, washed with EtOH and ether and allowed to dry overnight in a vacuum desiccator to yield 40 (0.45 g, 1.05 mmol, 84 %) as a yellow-brown solid: mp >400oC; 1H NMR (100 MHz, d6-DMSO) δ (ppm) δ 8.357 (t), δ 7.978 (d, J = 8.4 Hz), δ 7.785 (t), δ 7.698 (d, J = 8.40 Hz), δ 7.280 (s); MS-ESI calcd 414.1 m/z, found 415.2 [M+H]; Anal. Calcd for

C18H12F2N6S2 * 1.3 HCl * 1.2 CH3CH2OH: C 47.38, H 4.00, N 16.25; Found C 47.48, H 3.61, N 15.86.

2,5-Bis(4-cyano-2-methoxyphenyl)thiazolo[5,4-d]thiazole (42)

4-Cyano-2-methoxybenzaldehyde 41 (1.77 g, 11 mmol) and dithiooxamide 35 (0.52 g, 4 mmol) were suspended in o-dichlorobenzene (80 ml) under a nitrogen atmosphere and heated at reflux with stirring. After 24h, the mixture was cooled, filtered, and washed with cold EtOH. The resulting solid was then dried under vacuum to yield crude 42 (1.04 g). The crude solid was then suspended in a 80:20 mixture of DMF-EtOH (~30 ml), boiled for about 4h and filtered hot. Once cooled, the mixture was filtered, washed with EtOH, and dried under vacuum with heat (~100oC) to yield 42 (0.94 g, 2.32 mmol, 54%) as yellow solid: mp >400oC; IR (cm-1) 3087, 2949, 2226, 1751, 1562, 1279, 1016, 821; Anal. Calcd for C20H12N4S2O2: C 59.39, H 2.99, N 13.85; Found C 59.20, H 2.84, N 13.84.

2,5-Bis(4-amidinophenyl-2-methoxy)thiazolo[5,4-d]thiazole HCl Salt (43)

Lithium bis(trimethylsilyl)amide (10.00 ml of 1.0 M in THF) was then added drop wise. The mixture was allowed to stir until completely dissolved. After a week, the mixture had not dissolved so a second addition of THF (5 ml) and lithium bis(trimethylsilyl)amide (10.00 ml) were added with continued stirring at room temperature. About 4 days later, a third addition of lithium bis(trimethylsilyl)amide (10.00 ml) was added. A week later a forth addition of lithium bis(trimethylsilyl)amide (12.00 ml) was added and allowed to continue stirring at room

temperature. After another 3 weeks or 7 weeks total, the mixture had become a red-brown solution. The flask containing the red-brown solution was then cooled to 00_{C for 30 min. The} solution was made slightly acidic by the drop wise addition of a saturated EtOH-HCl solution (~5 ml) and allowed to stir in the ice bath overnight. Following overnight stirring, the mixture was concentrated, suspended in anhydrous ether, and allowed to stir at room temperature for 30 min. The mixture was then filtered and washed with ether. The pale yellow solid was then dried under vacuum at 500_{C for 15 min.}

The crude salt was then converted into the free base by suspending it in H2O (~15 ml), placing it in an ice bath, and adding NaOH (10%) dropwise until the solution was slightly basic. The mixture was then filtered with cold H2O and stored overnight in a vacuum desiccator. The resulting solid was suspended in an 80:20 mixture of EtOH-MeOH (~18ml) and allowed to stir for 10 min. For conversion into the HCl salt, saturated EtOH-HCl (~4 ml) was added until the suspension was slightly acidic and it was allowed to stir overnight at room temperature. The precipitate was filtered, washed with EtOH and ether and allowed to dry overnight in a vacuum desiccator to yield 43 (0.88 g, 1.49 mmol, 112 %) as a yellow-green solid: mp >400oC; 1H NMR (100 MHz, d6-DMSO) δ (ppm) 7.287 (1H, s), δ 7.816 (1H, d, J = 8.4 Hz); δ 7.996 (1H, d, J = 8.4

Hz); MS-ESI calcd m/c 438.1, found m/z 439.3 [M+H]; Anal. Calcd for C20H18O2N6S2 * 1.4 HCl * 0.9 CH3CH2OH: C 49.31, H 4.71, N 15.83; Found C 49.43, H 4.32, N 15.42.

Molecular Modeling

Compounds were constructed in SYBYL 8.1 and then minimized to convergence using the conjugate gradient method with a termination gradient of 0.001, Tripos force field, and

Gasteiger-Huckel charges. Grid Search, of the Advanced Computation module in SYBYL, was then employed to systematically explore torsional freedoms and acquire the lowest energy conformations. All four torsional angles of the compounds were examined in full, 360°, through increments of 60 degrees. At each increment the bond is constrained and the conformation is minimized.

4. FLUORESCENCE